This invention relates to charged particle beam lithography and, more particularly, to a new and improved system including a new and improved magnetic electron lens system containing deflection coils.
FIG. 1 illustrates a typical prior art system 10 having a composite objective lens 12. In this Figure, there is provided a particle (electron or ion) source 14 from which is formed a beam 16 directed through a number of apertured plates 18 and blanking and focusing lenses 20 until the beam reaches the composite objective lens 12 for directing the beam onto a target 22. Conventionally, the target 22 is a resist layer positioned on a stage 24 which moves in a direction at right angles to the deflection of the beam. For more detailed information on this writing technique, see the U.S. Patents to Collier, et al. and Lin, Nos. 3,900,737 and 3,801,792, respectively.
FIG. 2 illustrates an enlarged cross-sectional view of the prior art objective lens 12 of FIG. 1 and shown to comprise a pair of deflection coils 26 and 28 within the bore 30 of the lens. The axial focusing field B.sub.Z (z) is formed by one or more gaps 32 located between a pair of cylindrical pole pieces 34 and 36 conventionally of Ferrite material located within a solenoidal excitation lens coil 38. To complete the magnetic circuit, a cylindrical outer return yoke 40, usually made of iron, surrounds the excitation coil 38. The yoke as shown also encompasses the pole pieces by flanges 42 to position the pole pieces.
Deflection coils located with the bore 30 of the lens and surrounded by the Ferrite pole piece cylinders 34 and 36, provide a superimposed axial lens field B.sub.Z (z) and a lateral time dependent deflection field B.sub.xy (z,t) which focus the beam 16 on and deflect the beam across the target resist layer 22 located below the lens.
To avoid aberration (degradation of focus) and distortion (nonlinear deflection) it is necessary to carefully design the deflection field B.sub.XY (z) to match the axial focusing field B.sub.Z (z) so that the shapes of the magnetic fields generated by the lens and deflectors match over the full range of lens and deflection excitation currents. If, for any reason, the shape B.sub.Z (z) of the focusing field changes non-linearly with lens excitation, then the system's aberration will be optimized for only one value of primary beam voltage.
The shape of the focusing field depends upon the shape and magnetic permeability of the pole pieces. If the permeability of the pole pieces is very high, then the field shape is dominated by gap geometry. However, as the lens excitation increases, the magnetizing force H increases and the permeability decreases. This effect, known as saturation, causes magnetic flux to leak out of the pole piece and into the lens bore, causing the shape of the focusing field to change. In the case of Ferrite pole pieces where the permeability is high only over a relatively narrow range of magnetizing force, this effect is noticeable even at low excitation. Due to the critical nature of the deflection optimization one would ideally maintain the Ferrite material at constant permeability over a wide range of excitation.
The traditional solution to the problem is to provide very thick pole pieces, i.e., thicken the pole pieces 34 and 36 so as to provide a large cross-sectional area, to carry the magnetic load. With a sufficiently large cross section, the Ferrite material does not approach saturation. This solution has several disadvantages. As well as increasing the weight, size and cost of the lens, it does not avoid the fact that the permeability of the Ferrite material changes dramatically over the operating range, resulting in small but significant changes in field shape. Since the iron portion of the lens is further removed from the bore and has a far higher saturation flux, its properties play a ralatively minor role.
Since the Lin patent, supra, shows a composite lens, a brief explanation of its deficiency as prior art is in order at this point. Reference is made to FIG. 3 which is a reproduction of the FIG. 3 of the Lin Patent.
The deflection system 60 is enclosed by a Ferrite cylinder 62 but is not shown as magnetically connected to the iron lens cylinder 52. In this configuration, the Ferrite is being used to provide a return path for the deflection field but, since only a single cylinder is used, the Ferrite is only one of two pole pieces necessary to form a lens field. It differs from the following disclosure insofar as the deflection system cannot be located within or below the lens gap. Further, no mention is made of using the iron gap as a means of reducing saturation in the Ferrite. As drawn the design would suffer from the saturation effects mentioned above.
It is an object of this invention to accommodate a low saturation material within the lens field without significantly affecting the linear relationship between the axial field strength and the excitation current at all points along the axis of the lens.
It is also an object of this invention to provide a new and improved particle beam lithographic system such as shown in FIG. 1 by incorporating a new composite objective lens.